Private branch exchange telephone system



Oct. 23, 1951 J. BowNE 2,572,104

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM /A/f/EA/TOR l.. J. BOWNE ATTORNEY Oct. 23, 1951 J, BOWNE 2,572,104

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM Filed April 29. 1949 2v SHEETS- SHEET 2 /Nl/E/VTOR j L. J Bow/VE ATTORNEY Patented Oct. 23, 1951 PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM Langford J. Bowne, Saugerties, N. Y., assgnor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 29, 1949, Serial No. 90,455

8 Claims.

This invention relates to telephone systems and particularly to improvements in circuit instrumentalities employed at private branch exchanges in completing telephone connections between local extension stations, and in renderingr local extension lines and trunk lines mutually accessible for the completion or extension of telephone connections between local and remote stations.

It is the object of this invention to simplify and otherwise improve cord circuits and attendants telephone and dial circuits of the type involved in the extension and completion of telephone connections between extension lines of a private branch exchange and betweensuch lines and trunk lines leading to a central office of either the manual or machine switching type.

A feature of the invention resides in a novel and simplified circuit arrangement for conditioning an attendants cord for automatic discrimination between inward and outward calls in order to provide non-through supervision on inward calls and through supervision on outward calls. More particularly, a calling supervisory relay in the cord which, on a connection between two local stations, functions under the control of switchhook contacts to effect the display of supervisory lamp signals, also functions under the control of an answering supervisory relay in the cord on a trunk connection to connect to, or preclude the connection to the cord of a holding bridge in accordance with whether the involved trunk call is on an inward or an outward basis.

Another feature of the invention which reduces the time of operation of the attendants dial circuit and otherwise contributes to its simplicity of design resides in the use of normally closed oinormal contacts of the dial to short-circuit the winding of a local relay which, incident to the movement of the dial from normal position and the consequent separation of the ofi-normal dial contacts, operates immediately to Short-circuit the attendants receiver and to otherwise condition the attendants telephone and dial circuit for dialing. Heretofore, dial oit-normal contacts were normally open and effected a circuit closure when the dial was moved out of normal position Y so that the circuit changes resulting from such circuit closure were attained only after the oi- 'normal dial spring `traveled the distance necesi operation.

A furtherfeature of the invention permitsv splitting the dial circuit when secrecy is desired on a trunk connection Iwhile the attendant is talking to the party at a local station. More particularly, when a talk and dial key is operated by the attendant, conversations may be had between the attendant and either of the interconnected parties, but if it is desirable that the distant party be excluded from the conversation a splitting key is operated to effectively split the cord and to connect a holding bridge to the trunk end of the cord while maintaining the attendants telephone circuit connected across the station end of the cord.

These and other features of the invention Will be better understood from the following detailed description when read in connection with the accompanying drawings, in which: l

Fig. l is a circuit schematic showing two local P. B. X lines L and L1 terminating in jacks at the P. B. X and a, block C-TD indicated as representing an attendants cord, and telephone and dial circuits. Y Fig. 2 is a detailed circuit diagram of the cord and attendants telephone and dial circuits which are indicated schematically at C and TD in Fig. 1; and v Fig. 3 is a circuit diagram of a trunk circuit by means of which calls incoming from and directed to a remote exchange may be completed or eX- tended at the P. B. X.

Local connections On a local connection involving stations A and B, Fig. l, and the corresponding lines L and L1 which terminate at the private branch exchange in jacks lli and l2, respectively, either station may initiate the call. For descriptive purposes it will be assumed that the subscriber, or party at station A is the calling party and desires to communicate with the party at station B. For simplicity of disclosure jack i2 associated with the line L1 is shown in simplied form.

To originate a call the party at station A removes the telephone from its support Iwhich closes the station, or line loop lin well-known manner. Incident to this operation by the calling party line lamp I3 is lighted as a call signal in a circuit which may be traced from grounded battery, through the iilament of lamp i3, normal contacts I4 of jack IQ, over the closed line loop including the tipV and ringr conductors of line L and the closed switchhook contacts at station A, jack contacts l5 to ground. If desirable an audible signal, such as a buzzer may be included in Circuit -vious. -not light at this time because its energizing cir- The cord circuit C (Fig. 2) is normally dry,

that is, there is no battery and ground connected to the tip and ring conductors thereof until the right cord plug I6 is inserted in the station jack. With plug IE in jack I0, sleeve relay I9 of the cord operates in a circuit which may be traced from grounded battery, over the winding of sleeve relay I9, outer upper armature and back contact of relay 20, normal contacts ZI of the night and through dialkey 22, sleeve contact of plug I6, sleeve I1 of jack I0, jack contacts I8 toground.

At its back contacts and outer upper and lower armatures relay I9 splits the cord. The right and left plugs IE` and are now interconnected by way of condensers 23 and 24, one circuit being traced from the tip contact of plug IE, and contacts 26 of ring right key 21, outer upper armatureand front contact of relay I9, condenser-24, front contact and inner lower armature of relay I9, contacts 230i talk and dial key 30, contacts 29 of ring left key 3I, to the ring contact of plug 25, and the other circuit being traced from the tip contact of plug 25, contacts 32 of key-3I, contacts 33 of key 30, inner upper armature and front contact of relay I9, condenser 23, front contact and outerlower armature of relay I9, condenser 38 and the parallelly-connected Windingl of answering supervisory" relay 34, conta'cts-35of key 21, to the ring contact of plug Talking battery and ground through the lower and upper windings of relay are now connected to the right, or station end oi the cord -by way-of the front contacts and outer lower and upper armatures of relay I9, the winding of answering supervisory relay 34 and the contacts -26 and 35 of'lkey 21.

Battery and ground arel also connectedL to the left or trunk and stationv end of the cord through the windings of calling supervisory relay 4B. This connection may be traced from grounded battery, lower winding of relay 40,

liront contact and inner lower armature of relay .I9, contacts' 2B of key 3D, contacts 29 of key 3I, .to the ring` Contact of plug 251, and from the vtip contact of plug 25, contacts .32 of key 3|, contacts 33 of key 30, inner upper armature and front contact of relay I9, upper winding of relay 40,. back contact and outer lower armature of Vrelay.2Il, to. ground.

The battery and ground connection is reversed at condensers 23 and 24 to improve transmission `by reducing` crosstalk which may leak through relays 36 and 40. The circuit through relay 35 islreversed relative to that through relay 40 to further reduce the possibility of crosstalk between adjacent cord circuits.

When the plug I6 was inserted into jack I0 of 'the calling line L, the same ground which caused f relay I9 to operate causes the lighting of lamp 4I associatedwith the trunk end of the cord. -The circuit in which lamp 4I is lighted isrob- Lamp 42 associated with plug IB does :ing supervisory relay 4i) operates.

cuit is open at the back contact of relay 34, which relay is now operated over the closed station loop including the line conductors of line L.

After the insertion of plug I6 into jack I0 the attendant actuates the talk and dial key 30 to connect her telephone set 44 across the cord `in order to communicate with the calling party at station A. With key 30 operated, talking battery for the attendant's transmitter is supplied over a circuit extending from grounded battery, over-the lament of resistance lamp 45, lower winding of retardation coil 46, connector contacts 41, transmitter 48, connector contacts 49, upper winding of retardation coil 45, lower off-normal contacts 5I of dial 50, which contacts normally short-circuit the winding of off normal relay 52, connector contacts 53 and 54, contacts 55 of key 30 to ground.

It will be observed that the attendants transmitter 48 is connectedalso across the primary, or right winding of induction coil 51 by way. of the monitoring key 58 and condenser 59 so that any speech activation of the transmitter 43 (with key 58 in its normal unoperated position) sets up corresponding speech frequency signals in the primary of coil 51 which are induced in the secondary winding of the coil for transmission to the calling station A by way of a circuit which may be traced from the upper terminal of the secondary winding of coil 51, connector contacts 5U andBI, contacts 62 of key 30, inner upper armature and front contact of relay I9, condenser 23, front contact and outer lower armature of relay-I9, condenser 35 and winding of answering supervisory relay 34` in parallel, contacts 35 of key 21, ring contact of plug I5, ring contact of jack I6, closed line loop at station A by way of the line conductors of line L, tip contact of jack ID, tip contact of plug I6, contacts 26 of key 21, outer upper armatureand front contact of relayY I9, condenser 24, front contact and inner lower armature of relay I9, contacts 63 of key 3D, connector contacts '64 andf65, condenser 6E, to the lower terminal of the secondary winding of coil 51.

The attendants'receiver 61 is connected directly across-the center section of the secondary winding ci coil 51 by way of connector contacts 68' on one side and on the other side `by connector contacts 69;':15 and 1I, contacts 12 of vkey `3l), and connector contacts 13 and 14, so kthat any speech signals emanating from station A are reproduced inthe receiver.

' Having connected'the telephone set 44 across 'the cord the P. B. X attendant converses with the calling party at station A and learns that a connection with station B on line L1 is desired. Accordingly, plug 250i the cord C is inserted intojackv I2 (Fig. 1) associated with line L1 on which the called station B isv located and the ring left key 3l is actuated. With key 3I operated, ringing current source 15 is connected across the tip and'ring terminals of plug25 by way of-alternate contacts of the key and associated .connected contacts, and current therefrom is projected out overline L1 to energize the audible signaling device, or ringer at the 4called station B.

When the telephone at the called station B is removed from its support in response to the audible call signal and after the attendant has restored the ring left key 3| to normal, call- The operating circuit for relay 40 extends from grounded battery, over the lower winding of relay 40, front contact rand inner lower armature of relay I9, contacts 28 of key 30, which key may be restored before or after signaling current is projected out over the called line, contacts 29 of key 3|, ring contacts of plug 25, ring contacts of jack I2 of the called line Li, over the closed line loop at station B, tip contacts of jack I2, tip contacts of plug 25, contacts 32 of key 3|, contacts 33 of key 30, inner upper armature and front contact of relay I9, upper Winding of relay 40, to ground at the back contact and outer lower armature of relay 20. At its uppermost armature and back contact relay 40 opens the energizing circuit for lamp 4| causing this lamp to be extinguished. Other armature operations of relay 40` are of no significance at this time.

The calling and called stations A and B are now connected for communication and the parties thereat may converse. The called station is supplied with talking battery by way of the windings of relay 40 whereas battery for the calling station is supplied through the windings of relay 36. The talking path between the interconnected stations is by way of condensers 23 and -24 as hereinbefore indicated.

Either station may recall the attendant by operating the Ystation switchhook. This causes the release and reoperation of the corresponding relay 34 or 40 and the ashing of the correspending lamp 42 or 4|. The attendant answers such signals by operating the talk and dial key 30.

At the termination of conversation between the parties at stations A and B the telephones thereat are returned to their supports whereupon the corresponding switchhook contacts are opened causing relays 34 and 40 to release. Lamps 42 and 4| therefor, are lighted as disconnect signals. `Cord plugs I6 and 25 are then removed from jacks and |2, respectively, and the circuit restores to normal.

Trunk connections-incoming A trunk circuit T is shown in Fig. 3 terminating'at the P. B. X in a jack |00. This circuit is equipped with thermistors |0| in the ring-up relay circuit to provide a time delay and thereby prevent false line signals when central office equipment functions after a connection istaken down at the P. B. X. These elements normally are of extremely high resistance but when ringing current is applied for approximately onehalf second' or longer the resistance of the thermistor connected to Athe side ofthe trunk to which ringing current is connected is reduced materially thus providing an operating circuit for relay |02 from either side of the trunk to ground.

Varistor |03 is provided for two purposes; (1) to provide a low resistance operating path for the thermistor, and (2) to shunt relay |02 on one-half cycle of the ringing current so that the relay will be operated steadily during the ringing period on the other` one-half cycle. The varistor |03 is so poled that the operating and locking circuits of relay |02 are aiding.

On an incoming call from a central ofce indicated at ||0 in Fig. 3, ringing current is applied'to either or both of the conductors of the trunk T in well-known manner. This current traverses the thermistors |0| and the associated condensers and thence to ground by way of the winding of relay |02 andthe shunting varistor |03 -and the corresponding connector contacts 6 |04 and |01. Relay |02 is energized on alter; nate half cycles of this current and operates.

At its upper armature and front contact, relay |02 completes an obvious energizing circuit for lamp |06 which includes the connector contacts |05 and |01. At its lower armature and front contact, relay |02 completes its own locking circuit which may be traced from grounded battery, over connector contact |08, resistance lamp |09, contact of jack |00, resistance ||2, lower armature and front contact and winding of relay |02, connector contact |04, to ground.

Upon noting the lighted lamp |06 the P. B. X attendant. inserts the left plug 25 of the cord C into the trunk jack corresponding to the lighted lamp. This results in the separation of jack contacts in the locking circuit for relay |02 whereupon this relay releases its armatures and opens the energizing circuit for lamp |06. Lamp |06 accordingly, is extinguished. Auxiliary jack contacts ||5 are closed rendering the ring conductor of the trunk continuous to the corresponding jack contact. Auxiliary jack contacts IIS also are closed incident to the insertion of plug 25 into the jack |00 and connect battery through resistance lamp |09 to the sleeve te'rminal of the jack.

The reason for employing a normally open ring connection between the ring contact of the trunk jack and the corresponding trunk conductor is because outgoing calls require that when the trunk plug is inserted in the jack the ring spring shall be open until the tip spring starts to move to avoid a possible short circuit of the tip and ring springs while the plug is entering the jack. This prevents a false pulse to the central oice equipment. When the plug is fully seated in the jack the tip and ring conductors are extended to the cord in the usual manner.

With plug 25 in jack |00, relay 20 operates in a circuit which is traced from grounded battery (Fig. 3), connector contacts |08, filament of resistance lamp |09, auxiliary jack contacts IIB, sleeve terminals lll of jack |00, sleeve terminal of plug 25, contact ||8 of night and through dialing key 22, Winding of relay 20, to ground.

Relay 20, at its inner lower armature and front contact, connects retardation coil |20 and resistance |2| in series across the cord to trip machine ringing and to provide a bridge for holding the central oiiice connection.

At itsinner upper armature and front contact, relay '20 connects the windings of calling supervisor-y relay 40 in series with resistance |22 so that relay 40 is used as a local circuit relay under control of the answering supervisory relay 34 as will be described morefully hereinafter. At its outer lower armature and back contact, relay 20 disconnects ground potential from the winding of relay 40, and at its outer upper armature and back contact it opens the circuit to sleeve relay I9 to prevent this relay from operating later when the connection is extended to the called stations.

It will be observed that the trunk plug 25 may be inserted in the trunk jack |00 either with the talk and dial key 30 operated or released.. If the key is unoperated, the bridge including re tardation coil |20 and resistance |2| is connected across the connected trunk incident to the opera@ tion of relay 20, by way of the back contact and middle armature of relay 40, contacts 28 of key 30, contacts 29 of key 3|, to the ring terminal of plug 25, andby way of the front contact and inner lower armature of relay 20, contacts 33 of key,30,fcontacts;32 of key3| to.-,t he,tip terminal of pluge.- `If keyf30 is operated, contacts-28 and 33fthereof are open and the bridge circuit is completed by'wayfoifV contacts 63 andi-62-of key 30, f lthe circuits AtromV these` points including the key; contacts A63, connector contacts A|56 and 65, backfcontact and outer-lower armature of relay |30,;connector contacts |54, dialing contacts of dial50, connector contacts|55, |3| landil, contacts |33 of key ,30,contacts- 29 ofkey 3| to the ringterminal of plug 25, andthe contacts B2 of key 39, connector terminals 6| and 60, outermost upper armature and back contact of relay |30, connector contacts |33 and l |35, contacts |36. ofkey 30, contacts 32 of key 3.I to the tip contact of plug25.

With plug 25 inyjack |03 and the talk and dialk key operated, the PV. B. X attendant may converse with Athe calling party1 to ascertain Vwith which line -a connection is desired.

As hereinbefore described talking battery for the attendants transmitterxfi is `supplied from therbattery associated with the resistance, la'rnp n35 and ground at contacts 55 of key 30 by way of the-ofmorrnal contacts 5I of dial 50 and connector contacts 53 and 56. Also, the attendants receiver Si is connected across the middle section of the secondary winding of induction coil 5'! so that the talking path to the trunk T may be traced from the upper terminal of the secondary coil 5'?, outermost upper armature and back contact of relay |30, connector contacts |34 and |35, contacts I3'of key 30, contacts 32 of key 3| to the tip terminal of plug 25 and out over the tip lead of the established connection to the calling station, back over the corresponding ring lead, ring contact of plug 25, contacts 23 of key 3|, contacts |33 of key 3B, connector contacts E3?. and I3I, connector contacts |55, dial contacts of dial 50, outer lower armature and .back contact of relay I 30, condenser 63, to the lower terminal of induction coil 5?.

Assuming the calling party requests that a connection be extended to line L on which station A is located, the attendant inserts the station -plug I6 into jack I0 which terminates the `called line L at the P. B. X, and actuates the ring right key 21.

The cord of this invention is arranged for auto- -matic discrimination between inward and outward calls in order to provide non-through7 supervision on inward calls and through supervision on outward calls. When ring right key 2l is actuated, relay |33 operates in a circuit whichinoludes grounded batter* winding of relay I3?, contacts |38 of key 2T, sleeve contacts of plug Iii and jack I3 (Fig. 1), jack contacts i3 and ground. Lamp 02, at this time, is lighted in a circuit which may be traced from grounded battery 2, associated connector contacts, la-

ment of lamp 42, back contact and armature of relay 34, and over the now grounded sleeve contacts of plug I5 and jack I9. At its lower armature and front contact, relay |37 completes a locking circuit for itself to ground on the sleeve of plug IG by way of contacts 2| of the night and through'dialing key 22.

At is outer upper armature and back contact, relay |37 opens a locking circuit for relay il so that the holding bridge will be reconnected to prevent the release of the central olce connection at the end of the conversation between the calling and called parties. This providesfor the station recalling the P. B. X attendant without releasing the central office equipment. Onoutward .callsfrelay I 3'I-does not operatefbecausekey 27 is -not operated, so that at the end of the conversationrelay 40, being locked up, as described under Local connections, the central cnice equipment will release immediately when the calling station disconnects.

v Atvrcontacts |40 and |4I of key 2l the ringing current sourcefl` is connected across the tip and ring conductors of the cord and thence across the corresponding conductors of the called line L.Y Ringing current from this source therefore is projected over the `called line to actuate the audible signal device, or ringer at the called station A.

When the called party answers in the usual manner and the key 27| is restored, relay 34 operates in series with the called station from battery at thecentraloilce. At its back contact, relay 34 opens the energizing circuit to lamp 42 causing the lamp to be extinguished. At itsfront contact, relay V31% .completes anoperating circuit for relay 40 which may be traced fromgrounded battery, lower winding of relay 40, resistance |22, front contact and innervupper armature of relay 20, upper winding of .relay 40, front contact, and armature of relay 34, sleeve contacts of plug I6 and `jack I0, jack .contacts I8 to ground. Relay 40 operates in this circuit but does notlock since its locking circuit is open at the back contact and outerupper armature of relay |3`I, which relay, as previouslydescribed is locked operated to ground on the sleeve terminal of the called line jack I0.

Relay 40, at vits middle armature and back contact, removes a short from resistance |32 thus increasing the resistance of the bridge across the cord, which bridge includes the retardation coil |20 and resistance |2| as previously indicated. This increase in the -resistance of the holding bridge is c ected when the called station answers so that later, when the,v stationY disconnects, there will be no open interval given to the central ofce While relays 34 Aand 40 release and `reconnect the retardation coil |20 across the cord to hold the central oice connection.

With the telephone removed from its support ai; station A, telephoneA communication between the `cal-ling and called parties may take place. Under this condition the tip conductor of the .cord C may-be traced from the tip contact of plug 25, over the cord C may be traced from the tip contactl of plug ,25, over the contacts 32 of key 3|, contacts 33 of key 30, back contact and outer .upper armature ofrelay. I9, contacts 26 of key 2l, andthe tip contact of plug I6; the ring conductor may be tracedfrom the ring contact of plug 25, contacts 29 of key 3|, contacts 28 of key 30,`back contactl and outer vlower armature` of relay I9, condenser 38 and the parallelly connected winding of relay34, contacts 20 of key 21 to theringfcontact of plug IB. Across the tip .and ring is connected the .high impedance hold- :ing Abridge which includes retardation coil |20 andl resistances i2i and |52., `Under thisA condition `talking battery for the called station A is tion Av with secrecy, this may be accomplished by the operationof both the talk and dial. key J3i) and the splitting key |50 .in thefattendants Ytelephone and dial-circuit TD.Y With key 30 voperated, contacts' 33 and 28 are open thus splitting the cord;atcontats 62 and 6,3 oikey30 the station end of the cord and the connected station A are connected to the coil 51 in the attendants telephone circuit thus permitting conversation between the attendant and the party at station A to take place.

At contacts 55 of key 30, ground is extended over connector contacts 54 and 53, over the left alternate contacts of key |50 to battery and ground through the winding of relay |30. Relay |30 operates in this circuit and, at its innermost upper armature and front contact, locks to ground at contacts 55 of key 30 independently of the splitting key |50. Talking battery for station A, under this condition, is provided through the upper winding of retardation coil |52. The circuit involving this battery supply may be traced from grounded battery, over the inner lower front contact and armature of relay |30, lower winding of retardation coil |52, connector contacts 65 and 64, contacts 63 of key 30, and thence over the established connection to station A and back to contacts 62 of key 30, connector contacts 6| and 60, upper winding of retardation coil |52, middle upper armature and front contact of relay |30 to ground.

The bridge holding the central oiiice end of the connection may be traced from the tip contact of plug 25, contacts 32 of key 3|, contacts |36 of key 30, connector contacts |35 and |34, right alternate contacts of key |50 (these contacts normally short-circuit the upper winding of retardation coil front contact and lower armature of relay |30, dialing contacts of dial 50, connector contacts |55, |3| and |32, contacts |33 of key 30, contacts 29 of key 3|, to the ring contact of plug 25. Thus the two ends of the cord are isolated from each other, battery supply and the attendants telephone circuit are associated with the station end of the cord and the central office end of the connection is held. The attendant may converse with the party at station A without the calling party hearing the conversation.

The attendant may reconnect the station A to the trunk T by restoring the splitting key |50. Under this condition, the talking circuit is through retardation coil |5| until the talk and dial key 30 is restored. This talking circuit may be traced as follows: the trunk end of the circuit extends from the tip contact of plug 25, contacts 32 of key 3|, contacts |36 of key 30, connector contacts |35 and |34, upper winding of retardation coil I5 i, front contact and outer lower armature of relay |30, connector contacts |54, dialing contacts of dial 50, connector contacts |55, |3| and |32, contacts |33 of key 30, contacts 29 of key 3 to the ring contact of plug 25; the station end of the talking connection may be traced from the tip contact of plug I6, contacts 26 of key 21, outer upper armature and back contact of relay I9, contacts 62 of key 30, connector contacts 6| and 60, outermost upper armature and front contact of relay |30, lower winding of retardation coil |5|, condenser |56, connector contacts 65 and 64, contacts 63 of key 30, back contact and outer lower armature of relay I9, winding of relay 34 and parallelly connected condenser 38, contacts 35 of key 21, to the ring contact of plug |6.

When the talk and dial key 30 is restored, the talking circuit between the called line L and the trunk T is through cord C, the! key contacts |36, 62, 55, 12, 63 and |33 of the key 30 being open thus disassociating the attendants telephone and dial circuit TD from the cord.

When the party at station A disconnects at the termination of conversation, relay 34 restores its armature causing lamp 42 to be lighted in a circuit which finds ground at contacts I8 of the station jack |0 and battery at 2. Relay 40 releases incident to the release of relay 34 causing resistance |42 to be short-circuited and reconnecting 'the lower impedance bridge, including retardation coil |20 and resistance |2| across the trunk end of the cord to prevent release of the central oiice connection. This also permits station A to recall the P. B. XV attendant without releasing the central oiice equipment.

Upon noting the lighted condition of lamp 42, the P. B. X attendant removes the plugs |6 and 25 from the jacks |0 and |00 respectively, to restore the circuits to normal condition. When the plug I6 is withdrawn from jack |0, the locking circuit of relay |31 is opened and this relay releases. When plug 25 is withdrawn from jack |00, the centraloice connection is released in well-known manner and relay 20 in the cord restores its armatures.

Trunk connections (outgoing) to dial central ol'iice Outgoing trunk calls from a local station such as station A are originated at the station in the usual manner and as previously described under Local connections. Under this condition line lamp I3 is lightedas la call signal and is extinguished when the P. B. X attendant inserts plug I6 into jack i0 associated with lamp I3. Relay I9 operates in a manner previously described to supply battery and ground to the right cord and relay 34 operates over the station loop to prevent the right cord lamp 42 from lighting. When the left plug 25 is inserted in the trunk jack |00, relay 20 operates from battery over the sleeve of the trunk jack. It is understood that prior to the insertion of plug 25 into jack |00, the attendant operated the talk and dial key 30 to connect her telephone to the calling station and, learning of the desires of the calling party inserts the plug into the trunk jack.

Relay 20, operated, prepares relay 40 for operation in a local circuit; disconnects ground from the upper winding of relay 40; opens the circuit to relay I9; opens the circuit to the left cord lamp 4|; and connects the retardation coil |20 and resistance 2| across the trunk. Relay |9 releases and removes battery and ground from both the right and left cords and connects the tip and ring conductors of the left cord to the tip and ring conductors of the right cord. The calling station A is thus connected to the central oflice trunk in series with the winding of relay 34.

The attendant dials on o-utgoing trunk connections such as the one thus far set up, with the talk and dial key operated. The dial 50 accordingly is operated in the well-known manner in accordance with the connection desired by the calling party at station A.

It will be noted at this time that the winding of off normal relay 52 in the attendants telephone and dial circuit TD is normally short-circuited by the off-normal contacts 5| of dial 50. Thus when the dial is moved olf normal preparatory to dialing the desired number and the offnormal contacts are separated, relay 52 operates in a circuit which may be traced from grounded battery, lament of resistance lamp 45, lower winding of retardation coil 46, connector contacts 41, transmitter 48, connector contacts 49, upper winding of retardation coil 46, winding of relay i l 52; .connector contacts 53 and 54, contacts 55 of key'30to ground.

At its lower armature and iront contact, relay 52 completes an obvious operating circuit for relayl'lii which finds ground potential at contacts 55 of key 30. At its outer upper armature and frontzcontact relay 52 short-circuits the attendants receiver Bl to prevent the attendant hearing clicks on subsequent operation of the dial circuit. Atv its inner upper armature and front contact relay v52 connects resistance |65 in series with condenser |56 across the station end of the cord circuit. Further movement of the dial oil" normal closes the oi-normal contacts |5| of dial 5i) thus short-.circuiting the tip and ring conductors of the'dial circuit to provide the minimum resistance loop possible to the central oiiice. This circuit may'be traced from the tip contact of plug 25, over contacts 32 of key 3i, contacts! 36 of key 30, connector contacts |35 and |35, conductor |62, contacts |5| and the dialing contacts of dial 50, connector contacts |55, i3! and |32, contacts |33 of key 35, contacts 29 or key 3| to the ring contact of plug 25.

Relay |30, operated, locks to ground at contacts 55 of key 3i). At its upper outermost and outer lower farmatures and back contacts, relay |30- splits theright end of the dial circuit from the-left end leaving the attendants telephone circuit connected to station A on the right end .ofthe-zcord. YAtits outer lower armature and front contact, relay |36 connects the upper windingof retardation coil across the left, or trunk end of the cord. At its outermost armature and frontcontact, relay |39 bridges the lower winding of Vretardation coil |5| in lseries with condenser |56 across the station end of the cord and the induction coil 51. At its middle upper and inner lower armatures and front contacts, relay |30 connects ground and battery through the wind- 1 ings of Vretardation coil |52 to the station for talking purposes and signaling battery supply after the dial restores to normal. At that time the party" at station A and the P. B. X attendant can talk to the central cnice through retardation coil |5| Which then acts as a repeating coil. While the dial is off normal, resistance |50 is connected across the lower Winding of retardation coil |5| for purposes which will appear hereinafter.

While the dial restores to normal the dialing contact makes and breaks the pulsing circuit 4to the central cnice free of any series impedance or bridged capacity in the dialing circuit. When the dial restores to normal, it removes the short circuit across the tip and ring conductors of the dial circuit and the upper winding of retardation coil v|5| and replaces the short circuit about the winding-of relay 52 which relay then releases.

Relay 52 released, removes the resistance |55 from across the lower Winding of retardation coil |5|. This combination of the dial removing the short circuit from the upper winding of retardation coil |5| and relay 52 removing the resistance |60'from the lower winding of coil |5| prevents a false pulse being transmitted to the central cnice due to the impedance of the retardation coil being connected in series with the trunk.

Subsequent operation and release of the dial causes subsequent operation and release of relay 52. When the complete code has'been sentthe attendant may restore the talk and dial key if the callingl station remains onthe connection orthelattendant may wait vuntil Ythe calledparty answers and then ring the calling station'infthe manner previously described.

Theimpulses transmitted over "the trunk Tby the operation of dial 5|) control the operation of central office switches in the well-knownmanner which function to extend the connection to-the called line.

When the talkand dial key 30 is restored. ground is removed from contacts 55 of thekey thus'causing the release of relay f|30 'and the restoration of the attendants telephone arid dial circuit TD to normal condition. Under'thiscondition the talking circuit from'the trunk 'I tothe calling line L is over thefcord'circuitAC. l

At the termination of conversationV on the connection just described' the calling party vat'station A disconnects in the usual manner causing relay 35 to release and open the voperating'fcircuitto relay 45. Relay 49, however, is held locked to ground at contacts :|655 of key 32 by way of the outer upper armature and back contact of relay |37 and the front contact and innermost armature of relayAB. 'Thus relay il Adcesnot release so that "the holding bridgeincluding coil |20 and resistance -I2liA isnot reconnectedfto the cord. The central onice equipment thereforwill release immediately' When-station A disconnects. This resultsin through supervision. .Y

When relay 34 releases, lamp 2f is lighted to indicate to the attendant that conversationvv has been concluded and the connection may be taken down. Plugs IS and 251therefore,'areremoved 'from jacks'i and ISST-respectively, restoring the circuit to normal.

M Ofnitoring The normal connection of the attendants induction coil 5'1 provides maximum eiiiciency for talking to a station or a central cnice trunk. This would, howeven'cause an undesirable loss when the attendant monitors on an established connection due to the bridging loss vandfroom noise picked up by the transmitter--fll8i The monitoring key 58 opens the transmitter side of the induction coil 5T raising its impedance to a satisfactory value for monitoring andpreventing interference from room noise.

Sleeve protection Varistors |99 and |58 associated with .relays 25 and |9, respectively, are provided to reduce the potential which would be impressed on thecord sleeve when a plug is removed from a jack. These varistors are of the point contact or dry type. Ordinarily, the current flowing through the varistor when the associated relay is operated is very low (less than 0.1 milliampere). Resistances |91 and |95 are connected in series with the associated varistors to reduce the current now to a maximiun 40 milliamperes in case the battery supply, should, for any reason become reversed.

What is claimed is:

l. ln a telephone exchange system, calling and called subscribers lines each having a station thereon including a switchhook and contacts con'- trolled thereby, a trunk line, means comprising a cord circuit for interconnecting said calling and called lines and for extending a connection from said trunk line to one of said subscribers lines, a single calling supervisory relay, a single answer. ing supervisory` relay, each of said relays in said cord circuit, means whereby said relays are rcsponsive to switchhook operations at thestations of said called and calling lines, respectively,

' when said'cord circuit is used to interconnect said o 13 subscribers lines, a bridge circuit, I means controlled by said calling supervisory relay for connecting said bridge circuit across said cord circuit, means including said answering supervisory relay and said calling supervisory relay for causing said calling supervisory relay-controlled means to connect said bridge circuit across said cord circuitl means whereby said answering supervisory relay and said calling supervisory relay are responsive to switchhook operations at a called subscribers station on a connection eX- tended thereto from said trunk line by way of.7

said cord circuit.

2. In a telephone exchange system, a calling line, a station on said calling line having a switchhook and contacts controlled thereby, a called line, a station on said called line, a trunk line, a cord circuit for interconnecting said calling and called lines and for extending a connection from said trunkline to one of said calling and called lines, a calling supervisory relay in said circuit, means whereby said relay is vresponsive to switchhook control from a called station on a connection involving said calling and called lines and said cord circuit, an answering supervisory relay in said cord circuit, means whereby said answering supervisory relay is responsive to swtchhook control from a calling station on a connection involving said calling and called lines and said cord circuit and responsive to switchhook control from a called one of said stations on a connection extended thereto from said trunk line by way of said cord circuit, means controlled by said answering supervisory relay on the latter type connection for operating said calling supervisory relay in a local circuit, a bridge circuit, and means controlled by said calling supervisory relay for controlling the connection of said bridge circuit to said cord circuit.

3. In a telephone exchange system, a calling subscribers line, a station on said calling line, a called subscribers line having a switchhook and contacts controlled thereby, a station on said called line, a trunk line, a cord circuit for interconnecting said calling and called lines and for extending inward and outward connections between said trunk line and any of said subscribers lines, a calling supervisory relay in said cord c ircuit, means whereby said calling supervisory relay is responsive to switchhook control from a called station on a connection involving said calling and called lines and said cord circuit, an answering supervisory relay in said cord circuit means whereby said answering supervisory relay is responsive to switchhook control from a calling station on a connection involving said calling and called lines and said cord circuit and means whereby said answering supervisory relay is responsive to switchhook control from certain of said stations on inward and outward connections involving said subscribers lines, said trunk line and said cord, means controlled by said answering supervisory relay on connections involving said trunk line for operating said calling supervisory relay in a local circuit, means for completing a locking circuit for said calling supervisory relay, means whereby said completing means is effective only on outward connections completed over said cord circuit, a bridge circuit, and means controlled by said calling supervisory relay for controlling the connection of said bridge circuit to said cord circuit. A

4. In a telephone exchange system, local sta.- tion lines and trunk lines, a cord for completing telephone connections between said lines and 'fil comprising a trunk jack and a station jack, an attendants telephone circuit, a key in said cord having normally closed contacts which render said cord continuous and which, when said key is operated, function to split said cord, alternate contacts controlled by said key, when operated, to connect said attendants telephone circuit across both said trunk jack and said station jack, a holding bridge, a key in said attendants telephone circuit, and electromagnetically operated means, means whereby said electromagnetically operated means is controlled by said second key while said rst key is operated for disconnecting said attendants telephone circuit from the trunk jack of said cord and connecting said holding bridge thereto.

5. In a telephone exchange system, local station lines and trunk lines, a cord for completing telephone connections between said lines and comprising a trunk jack and a station jack, an

attendants telephone circuit, a key in said cordr having normally closed contacts which render said cod continuous and which, when operated, function means whereby said key splits said cord, alternate contacts, means whereby said contacts are controlled by said key when operated to connect said attendants telephone circuit across both said trunk jack and said station jack, a twowinding retard coil, a key in said attendants telephone circuit, a relay, an operating circuit for said relay, means for completing said operating circuit incident to joint operation of both said keys, contacts, means whereby said contacts are controlled by said relay for disconnecting said attendants telephone circuit from the trunk jack of said cord and connecting a bridge circuit thereto which includes closed contacts of said second key, means including said i'lrst key for locking said relay operated independent of said second key, and means including contacts of said second key when released for connecting one winding of said retard coil across the trunk jack of said cord and the other winding of said retard coil across said attendants telephone circuit and for opening the bridge circuit to said trunk jack.

6. In a telephone exchange system, a line circuit, a trunk circuit, means for interconnecting said circuits comprising a cord circuit, an operators telephone and dial circuit, a key in said cord circuit, contacts, means whereby said contacts are controlled by said key and adapted to connect said telephone and dial circuit with said cord circuit, a dial in said dial circuit including normally closed off-normal contacts, a relay normally shunted by the off-normal contacts of said dial. means including contacts of said key, means whereby said contacts are eiiective incident to the operation of said key for completing an energizing circuit to said telephone by way of the closed off-normal contacts of said dial whereby said relay is rendered operative incident to the movement of said dial from normal position and the consequent opening of its normally closed off-normal contacts, and means controlled by said relay, when operated, for short-circuiting the receiver of said telephone and for connecting f normal Vcontacts of said dial, means `for complet ing an energizing circuit to the transmitter of said telephone by way of the off-normal contacts of `said dial, means whereby said means is effective incident to the operation of said key, means whereby said relay is rendered operative incident to the movement of said dial from normal'position and the consequent separation of its normally closed oil-normal contacts, and means controlled by said relay, when operated, for shortcircuiting the receiver of said telephone.

S. A cord circuit for interconnecting subscribers lines and trunk circuits and including a trunk cord and a station cord, a plug terminating each cord and each including a sleeve Contact, a sleeve relay having its winding interposed betweenbattery andthe sleeve contact of the station plug, means whereby an energizing circuit for said sleeve relay is completed when said station cord plug is inserted in a jack having the sleeve terminal thereof at ground potential, a trunk relay interposed between ground potential and the sleeve contact yof the trunk plug, means whereby an energizing circuit for said trunk relay LANGFORD J. BOWNE.

REFERENCES CITED The following references are vofrecord in the m le of this patent:

UNITED STATES PATENTS t Number Name Date Nelson June 25, 1929 Labaugh Jan. 26, 1937 Welch Dec. 6, 1938 King Feb. 17,1942 Wallace .Dec. 22,1942 Bascom Sept. 10, 1946 

